In a conventional communication system in which a sensor transmits a detection signal to a control apparatus, the sensor and the control apparatus are configured to operate under the common time information. For example, as disclosed in US 2013/0343472 A1, the control apparatus generates a trigger signal as a request signal, and transmits the trigger signal to the sensor. In response to the request signal, the sensor transmits the sensor signal as a response signal to the control apparatus.
A microcomputer included in the control apparatus receives the sensor signal, which is transmitted from the sensor at a predetermined transmission cycle. Herein, the sensor signal is transmitted from the sensor as a digital signal. In the communication system, after the control apparatus receives the sensor signal, the microcomputer performs a control operation based on the value of the sensor signal at a predetermined operation cycle. Herein, the control operation is provided by arithmetic operation performed by the microcomputer. In this communication system, suppose that a transmission cycle of the sensor is equal to the operation cycle of the microcomputer. In this case, when the sensor and the microcomputer operate based on different timers, the transmission cycle of the sensor may be shifted from the operation cycle of the microcomputer, and a control performance of the microcomputer may be deteriorated.
Regarding the cycle shift between the operation cycle of the microcomputer and the transmission cycle of the sensor signal, US 2013/0343472 A1 discloses a configuration in which the sensor transmits the sensor signal to the microcomputer after the sensor receives a trigger signal transmitted from the microcomputer. With this configuration, the operation cycle of microcomputer and the transmission cycle of the sensor can be synchronized with each other. However, in this configuration, when the transmission of the trigger signal has an abnormality and a transmission failure occurs to the trigger signal, the sensor signal cannot be transmitted from the sensor to the microcomputer. As a result, the microcomputer fails to acquire any information about the detection target, and cannot perform the control operation. Failure of sensor information acquisition may cause a more adverse effect compared with the cycle shift between the operation cycle of the microcomputer and the transmission cycle of the sensor signal.